Hops have been used for centuries for the purpose of contributing a clean bitter taste to beer. Analysis of hop constituents has demonstrated that the most important bittering compounds derived naturally from hops are a group of isomers and analogous compounds generally known as alpha acids (humulones) and their derivatives isoalpha acids (isohumulones). Other derivatives [e.g., dihydroisoalpha acids, tetrahydroisoalpha acids (THIAA, tetrahydroisohumulones) and hexahydroisoalpha acids (HHIAA, hexahydroisohumulones)] are produced commercially.
In a conventional brewing process, hop cones are boiled with sweet wort in a copper kettle for about one to two hours, and then the wort is filtered and allowed to cool. Isoalpha acids are produced by isomerization of alpha acids during boiling of the wort. However, the isomerization yield in the boiling process is low and thus commercial processes have been utilized to extract the active compounds (alpha acids and beta acids) from the hops and convert them to the desired hop flavors (e.g., isoalpha acids, tetrahydroisoalpha acids) at relatively high yields. The commercially produced hop flavors may then be added post-fermentation so that they are not adversely affected during the wort boil.
In one commercial process, alpha acids are isomerized and reduced to dihydroisoalpha acids under basic conditions with a reducing agent such as sodium borohydride at elevated temperatures. In another commercial process, alpha acids are isomerized into isoalpha acids under basic conditions at elevated temperatures. Tetrahydroisoalpha acids are produced commercially by a multi-step route from beta acids, and hexahydroisoalpha acids are produced commercially by a reduction of tetrahydroisoalpha acids.
In addition, the literature teaches the hydrogenation of normal homolog isoalpha acids at a pH of about 3 resulting in low yields of tetrahydroisoalpha acid (P. M. Brown, G. A. Howard and A. B. Tatchell, J. Chem. Soc. 545 (1959)). That reference also teaches the hydrogenation with platinum oxide of normal homolog isoalpha acids at a pH of about 10 to give a low yield of isoalpha acids with only one double bond hydrogenated. The reference also teaches the hydrogenation of normal homolog isoalpha acids at a pH of about 3 to yield a deoxygenated THIAA. Another reference teaches the reduction of THIAA to deoxygenated products by hydrogenation with palladium on carbon in methanol at a pH of about 3 (E. Byrne and S. J. Shaw, J. Chem. Soc. (C), 2810 (1971)).
Although there are a variety of methods known to transform alpha acids to the desired hop flavor, there is a continuing search in this field of art for processes that are simple, inexpensive, and high yielding.